As the speed and complexity of electronic products increases, so do the challenges of manufacturing these products. The shrinking feature sizes of high-tech electronics are necessary to achieve the high clock speeds of digital electronics. This miniaturization is often achieved by adding more inner layers to PCBs and reducing the size of the circuit features. Miniaturization requires all manufacturing process tolerances to be reduced.
Originally, plated through holes in multilayers performed two functions, to interconnect layers, and to provide mounting for components. Surface mounted technology eliminated the need for the component mounting function. Therefore, the only purpose of the plated through hole is to interconnect the layers of the multilayer board.
The requirement for high speed circuitry dictates high density of conductors with, consequently, smaller traces and pads are designed. Printed circuit material is inherently unstable, therefore, small features drift during lamination. Laminating drift is getting bigger in proportion to the feature of the circuit board. For example, 0.002" drift in lamination represents a small percentage of a 0.030" diameter pad. However, the same 0.002" drift is very significant for a 0.010" pad. In addition, the annular ring of a 0.030" diameter pad with a 0.018" diameter hole will be 0.006" in contrast to a 0.010" pad with a 0.006" diameter hole which will be 0.002". This condition does not require absolute positioning accuracy since the components are not mounted through the hole. Instead, uniform drift of all the layers during lamination is necessary to line up all layers of the circuitry. The remaining task is to find the pad and determine the actual drift and drill the hole through it without breaking out of the annular ring.
Producing high density multilayer boards involves a lamination process. During the lamination process heat and pressure are applied to the stack of layers in order to bind it together. During this process, dimensional changes occur in the position of the circuitry in each layer. In most cases, special test coupons with fiduciary marks are designed into the boards in order to determine the amount of image shift during the lamination process. By comparing nominal locations with actual locations of the fiduciary marks, the amount of shift can be determined. The shifts of all points can be analyzed by computer to determine average correction in X, Y and theta axes to optimize the pattern and determine the best fit between the theoretical axes and the actual axes of the board.
The continually decreasing size of high density multilayer features applies more demands from registration accuracy. Hard tooling methods are no longer adequate for the accuracies required to ensure proper registration between circuitry details and drilled holes, or registration edges of the high density multilayers. External X-ray methods of determining shift are time consuming and not very accurate. Such methods typically employ an independent X-ray system, located apart from the drilling machine, which require removing the board from the drilling machine and do not provide adequate resolution. Also, correction numbers had to be fed into the controller of the drilling machine manually. This method is slow and not accurate enough. In addition, it requires transfer of the board from the drilling machine which introduces tooling errors to the process.
The object of the invention is to provide a practical means of determining lamination drift in the multilayers and apply appropriate corrections to obtain the best fit between the actual and theoretical pattern before drilling without removing the board from the drilling machine.